Ticket #3251: mcviewer: grab focus on mouse click in QuickView mode.

(mcview_t): add new member 'active' to recognize current state in QuickView mode.
(mcview_callback): toggle mcview_t::active in MGS_FOCUS and MSG_UNFOCUS
event handlers.
(do_mcview_event): use mcview_t::active to grab focus.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Merge branch '3251_quickview_grab_focus'

* 3251_quickview_grab_focus:
  Ticket #3251: mcviewer: grab focus on mouse click in QuickView mode.